Hardfacing of Iron and Steel Utilizing Cast Iron Scraps

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M_7 C_3 appearing in the Fe-C-Cr ternary system can be formed as a pure carbide and is expected to have a low melting temperature, a high hardness and excellent bonding strength with iron substrate. In the present work, microstructure and properties of bulk chromium carbide specimen obtained by melting raw materials have been investigated and it is found that the microstructure of carbide specimen with low chromium content consists of M_7 C_3 and ledeburite, while those of 50%Cr and 60%Cr specimens are obtained as a pure M_7 C_3 carbide. On the basis of these findings, an application of M_7 C_3 chromium carbide as a hardfacing of iron and steel was examined utilizing cast iron scraps as starting materials. Hardness of laser-clad carbide layr was increased with chromium content and it was revealed that the hardfacing layr containing 60%Cr subjected to laser surface melting consists of almost pure M_7 C_3 carbide and shows excellent wear property. Since these carbide was found to posess a coefficient of thermal expansion near those of iron and steel, the hardfacing layr showed a tight bonding with iron sustrate. In the case of utilizing cast iron scraps as a raw material, the same microstructure having a high hardness and an excellent wear resistance was obtained through one-pass laser melting and no significant deference was observed when using gray iron scraps and S.G.iron scraps. Some cracking was seen on the section of laser-hardfacing materials, running perpendicular to the interface between carbide and substrate. It was indicated that the coupled-diffusion processing for the modification of the microstructure of subsurfacc of the iron substrate to functionally graded microstructure can be effective to supress the residual stress concentration near the interface and hence cracking in the carbide layr.